The present invention relates generally to digital image recording devices, more particularly to digital cameras and digital video cameras, and even more particularly to the focus of such devices.
In order to record clear images of objects at varying distances from a digital image recording device such as a digital camera or digital video camera, the image projected onto the film or image sensor used for image capture must be in focus. A typical method to obtain such focus is to vary the distance between the lens and the plane of the film or image sensor either manually or automatically in relation to the distance from the object to the lens.
However, less expensive cameras can be produced using a fixed lens system. In such cameras, the distance from the lens to the plane of exposure is fixed. Now, however, obtaining a clear image becomes more difficult, as the camera must be moved to a fixed distance from the object at which the object is in focus. The focus object-to-lens distance is defined by the focal length of the lens system and the lens-to-plane-of-exposure distance, wherein the lens-to-plane-of-exposure distance is the distance between the lens of the digital image recording device and the plane of the film or image sensor used to record the image of the object. Among other means, the focus object-to-lens distance can be obtained and subsequently adjusted by either (1) guessing the distance, (2) using an optical range-finder, (3) forming two images on an image sensor from two lenses located a short distance apart, using a mirror to move one of the images until the two sets of signals produced by the two images on the sensor have maximum correlation, and then correlating the object-to-lens distance with the position of the mirror, (4) measuring the time taken for an ultrasonic wave pulse to reach the object and be reflected back to a detector on the camera, (5) measuring the amount of light or infra-red radiation reflected back from a scene from an emitted flash and correlating that measurement to the object-to-lens distance based on the average reflectance observed with scenes of average reflectance, and (6) viewing the image prior to exposure as in a single-lens reflex camera. All these methods are useful, but all have their disadvantages. Guessing is at best error prone, while the other techniques are expensive and time consuming as they may require additional equipment and several iterations of measurements with subsequent adjustment of the object-to-lens distance before making an exposure. In addition, false readings may occur in some situations depending upon the technique used. As an example, a heavily illuminated scene could indicate a false object-to-lens distance if that distance is set using the measurement of object reflected light from an emitted flash. Lens systems having a large range of object-to-lens distances in which the image of the object is in focus have also been used but are useful only when object-to-lens distances are large.
Thus, there is a need for an image exposure system that can easily, accurately, inexpensively, and automatically capture focused images using fixed lens systems.
In preferred embodiments, the present patent document describes method and apparatus for obtaining a focused image using a digital image recording device having a fixed lens-to-plane-of-exposure distance, wherein the lens-to-plane-of-exposure distance is the distance between the lens of the digital image recording device and the plane of the film or image sensor used to record the image. In a preferred embodiment, a digital image recording device, which could be a digital camera or a digital video camera, automatically records a set of successively detected images of an object at varying object-to-lens distances and identifies the image most in focus of the set of images recorded.
To obtain the set of images, either the camera or the object whose image is to be recorded is moved relative to the other. A preferred embodiment computes a contrast value for each stored image and chooses the image with the greatest contrast value to be the image most in focus. The other images are more blurred, thus having a smaller computed contrast value.
In an alternative embodiment, a distance determination device detects the position at which the image projected onto the image sensor is in focus and automatically enables recording of the image detected at that distance. The distance determination device could be a distance meter or similar device.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.